Immunolocalization of Na+,K(+)-ATPase and carbonic anhydrase in the gerbil's vestibular system

Does Otosclerosis Affect Dark and Transitional Cells in the Human Vestibular Labyrinth?

HYPOTHESIS

The density of vestibular dark cells (DCs) and vestibular transitional cells (TCs) can be quantitatively decreased in human temporal bones with otosclerosis.

BACKGROUND

Previous reports have shown that otosclerosis can lead to vestibular symptoms.

METHODS

We examined 61 human temporal bone specimens from 52 deceased donors with otosclerosis group-with and without endosteal involvement (EI), and with and without endolymphatic hydrops (EH)-versus 25 specimens from 18 age-matched controls. Using light microscopy, we evaluated the nonsensory epithelium of the lateral semicircular canal (LSC) and posterior semicircular canal (PSC) of the human vestibular labyrinth, focusing on the density of DCs and TCs.

RESULTS

In both the LSC and the PSC, as compared with the control group, the mean density of DCs significantly decreased in the EI (+) group, in the EI (+) and EH (+) subgroup, and in the EI (+) and EH (-) subgroup (p < 0.05). In addition, we found a significant difference in the mean density of DCs between the EI (+) group and the EI (-) group in the LSC and in the PSC (p < 0.05). But we found no significant difference in the mean density of TCs in any of the otosclerosis groups or subgroups as compared with the control group (p > 0.05).

CONCLUSION

We found a decrease in the density of DCs associated with EI in human temporal bone specimens with otosclerosis, regardless of the presence of EH. This decrease might cause damage in ion and water transportation, leading to vestibular symptoms.

Neurotransmitters in the vestibular system

Neuronal networks that are linked to the peripheral vestibular system contribute to gravitoinertial sensation, balance control, eye movement control, and autonomic function. Ascending connections to the limbic system and cerebral cortex are also important for motion perception and threat recognition, and play a role in comorbid balance and anxiety disorders. The vestibular system also shows remarkable plasticity, termed vestibular compensation. Activity in these networks is regulated by an interaction between: (1) intrinsic neurotransmitters of the inner ear, vestibular nerve, and vestibular nuclei; (2) neurotransmitters associated with thalamocortical and limbic pathways that receive projections originating in the vestibular nuclei; and (3) locus coeruleus and raphe (serotonergic and nonserotonergic) projections that influence the latter components. Because the ascending vestibular interoceptive and thalamocortical pathways include networks that influence a broad range of stress responses (endocrine and autonomic), memory consolidation, and cognitive functions, common transmitter substrates provide a basis for understanding features of acute and chronic vestibular disorders.

Distribution of Na,K-ATPase α subunits in rat vestibular sensory epithelia

The afferent encoding of vestibular stimuli depends on molecular mechanisms that regulate membrane potential, concentration gradients, and ion and neurotransmitter clearance at both afferent and efferent relays. In many cell types, the Na,K-ATPase (NKA) is essential for establishing hyperpolarized membrane potentials and mediating both primary and secondary active transport required for ion and neurotransmitter clearance. In vestibular sensory epithelia, a calyx nerve ending envelopes each type I hair cell, isolating it over most of its surface from support cells and posing special challenges for ion and neurotransmitter clearance. We used immunofluorescence and high-resolution confocal microscopy to examine the cellular and subcellular patterns of NKAα subunit expression within the sensory epithelia of semicircular canals as well as an otolith organ (the utricle). Results were similar for both kinds of vestibular organ. The neuronal NKAα3 subunit was detected in all afferent endings-both the calyx afferent endings on type I hair cells and bouton afferent endings on type II hair cells-but was not detected in efferent terminals. In contrast to previous results in the cochlea, the NKAα1 subunit was detected in hair cells (both type I and type II) but not in supporting cells. The expression of distinct NKAα subunits by vestibular hair cells and their afferent endings may be needed to support and shape the high rates of glutamatergic neurotransmission and spike initiation at the unusual type I-calyx synapse.

Expression and function of scleraxis in the developing auditory system

A study of genes expressed in the developing inner ear identified the bHLH transcription factor Scleraxis (Scx) in the developing cochlea. Previous work has demonstrated an essential role for Scx in the differentiation and development of tendons, ligaments and cells of chondrogenic lineage. Expression in the cochlea has been shown previously, however the functional role for Scx in the cochlea is unknown. Using a Scx-GFP reporter mouse line we examined the spatial and temporal patterns of Scx expression in the developing cochlea between embryonic day 13.5 and postnatal day 25. Embryonically, Scx is expressed broadly throughout the cochlear duct and surrounding mesenchyme and at postnatal ages becomes restricted to the inner hair cells and the interdental cells of the spiral limbus. Deletion of Scx results in hearing impairment indicated by elevated auditory brainstem response (ABR) thresholds and diminished distortion product otoacoustic emission (DPOAE) amplitudes, across a range of frequencies. No changes in either gross cochlear morphology or expression of the Scx target genes Col2A, Bmp4 or Sox9 were observed in Scx(-/-) mutants, suggesting that the auditory defects observed in these animals may be a result of unidentified Scx-dependent processes within the cochlea.

Critical roles of transitional cells and Na/K-ATPase in the formation of vestibular endolymph

The mechanotransduction of vestibular sensory cells depends on the high endolymphatic potassium concentration ([K+]) maintained by a fine balance between K+ secretion and absorption by epithelial cells. Despite the crucial role of endolymph as an electrochemical motor for mechanotransduction, little is known about the processes that govern endolymph formation. To address these, we took advantage of an organotypic rodent model, which regenerates a genuine neonatal vestibular endolymphatic compartment, facilitating the determination of endolymphatic [K+] and transepithelial potential (Vt) during endolymph formation. While mature Vt levels are almost immediately achieved, K+ accumulates to reach a steady [K+] by day 5 in culture. Inhibition of sensory cell K+ efflux enhances [K+] regardless of the blocker used (FM1.43, amikacin, gentamicin, or gadolinium). Targeting K+ secretion with bumetanide partially and transiently reduces [K+], while ouabain application and Kcne1 deletion almost abolishes it. Immunofluorescence studies demonstrate that dark cells do not express Na-K-2Cl cotransporter 1 (the target of bumetanide) in cultured and young mouse utricles, while Na/K-ATPase (the target of ouabain) is found in dark cells and transitional cells. This global analysis of the involvement of endolymphatic homeostasis actors in the immature organ (1) confirms that KCNE1 channels are necessary for K+ secretion, (2) highlights Na/K-ATPase as the key endolymphatic K+ provider and shows that Na-K-2Cl cotransporter 1 has a limited impact on K+ influx, and (3) demonstrates that transitional cells are involved in K+ secretion in the early endolymphatic compartment.

Endogenous ouabain in Ménière's disease

OBJECTIVE

Endogenous Ouabain (EO) has been demonstrated to modulate the activity of Na+, K+ -ATPase. Our purpose was to measure plasma levels of EO in Ménière's Disease (MD) subjects as a possible predisposing factor to developing and maintaining hydrops.

STUDY DESIGN

Case-control study.

SETTINGS

University hospital.

PATIENTS

Thirty-nine MD subjects and 29 controls with a lifetime negative history for vertigo and dizziness.

MAIN OUTCOME MEASURES

Plasma levels of EO.

RESULTS

Plasma EO in MD subjects was in the range between 33 and 504 pmol/L (median, 135.5 pmol/L), whereas in the control group, plasma EO varied between 70 and 724 pmol/L (median, 205 pmol/L). The Mann-Whitney U test detected a statistically significant difference (p = 0.0001).

CONCLUSION

Low plasma levels of EO have been proposed to augment Na-K pump activity, whereas high EO levels show an inhibitory effect on the pump activity. A proper pump activity may be necessary to keep the right ionic amount and osmolarity in endolymph. Although other possibilities may be considered, we suggest that altered control mechanisms of pump activity may be related to the pathogenesis and maintenance of MD.

Assembly of the otoconia complex to the macular sensory epithelium of the vestibule

In the inner ear, specificity of stimulus perception is achieved by associating the sensory epithelia of the three mechanoreceptor organs, the utricle/saccule, cristae, and cochlea, with distinct types of acellular matrices. Only the utricle and saccule have an extremely dense matrix, the otoconial complex, which overlies the sensory epithelium (macula) and provides inertial mass to generate shearing forces essential for the mechanoreceptors to sense gravity and linear acceleration. Such sensation is necessary for spatial orientation and balance. The importance of otoconia is clearly demonstrated by the impact of balance disorders upon the elderly population that involve otoconia degeneration, as well as by canalithiasis and cupulolithiasis, in which otoconia are dislocated. This underscores the need to understand how otoconia are formed and maintained and how to prevent their degeneration. To date, a number of otoconia-related proteins have been identified mostly in mice and bony fish. Although most of these proteins are also present in other structures of the inner ear, a distinct collection of proteins in the macula plus the unique ionic microenvironment of the endolymph near its epithelium likely contribute to the site-specific calcification of otoconia. Based on the current literature and ongoing research, this mini-review postulates a working model of how the otoconia complex is assembled specifically above the macular sensory epithelium of the vestibule. The central hypothesis of this model is that proteins are critical in sequestering calcium for crystallization in the calcium-poor endolymph. The review also sets forth some issues that need to be resolved in the future.

Deafness and cochlear fibrocyte alterations in mice deficient for the inner ear protein otospiralin

In the cochlea, the mammalian auditory organ, fibrocytes of the mesenchymal nonsensory regions play important roles in cochlear physiology, including the maintenance of ionic and hydric components in the endolymph. Occurrence of human deafness in fibrocyte alterations underlines their critical roles in auditory function. We recently described a novel gene, Otos, which encodes otospiralin, a small protein of unknown function that is produced by the fibrocytes of the cochlea and vestibule. We now have generated mice with deletion of Otos and found that they show moderate deafness, with no frequency predominance. Histopathology revealed a degeneration of type II and IV fibrocytes, while hair cells and stria vascularis appeared normal. Together, these findings suggest that impairment of fibrocytes caused by the loss in otospiralin leads to abnormal cochlear physiology and auditory function. This moderate dysfunction may predispose to age-related hearing loss.

Downregulation of otospiralin, a novel inner ear protein, causes hair cell degeneration and deafness

Mesenchymal nonsensory regions of the inner ear are important structures surrounding the neurosensory epithelium that are believed to participate in the ionic homeostasis of the cochlea and vestibule. We report here the discovery of otospiralin, an inner ear-specific protein that is produced by fibrocytes from these regions, including the spiral ligament and spiral limbus in the cochlea and the maculae and semicircular canals in the vestibule. Otospiralin is a novel 6.4 kDa protein of unknown function that shares a protein motif with the gag p30 core shell nucleocapsid protein of type C retroviruses. To evaluate its functional importance, we downregulated otospiralin by cochlear perfusion of antisense oligonucleotides in guinea pigs. This led to a rapid threshold elevation of the compound action potentials and irreversible deafness. Cochlear examination by transmission electron microscopy revealed hair cell loss and degeneration of the organ of Corti. This demonstrates that otospiralin is essential for the survival of the neurosensory epithelium.

Potassium recycling pathways in the human cochlea

OBJECTIVES/HYPOTHESIS

Potential pathways for recycling potassium (K+) used in the maintenance of inner ear electrochemical gradients have been elucidated in animal models. However, little is known about K+ transport in the human cochlea. This study was designed to characterize putative K+ recycling pathways in the human ear and to determine whether observations from animal models can be extrapolated to humans.

STUDY DESIGN

A prospective laboratory study using an immunohistochemical approach to analyze the distribution of key ion transport mediators in the human cochlea.

METHODS

Human temporal bones were fixed in situ within 1 to 6 hours of death and subsequently harvested at autopsy. Decalcification was accomplished with the aid of microwaving. Immunohistochemical staining was then performed to define the presence and cell type-specific distribution of Na,K-ATPase, sodium-potassium-chloride cotransporter (NKCC), and carbonic anhydrase (CA) in the inner ear.

RESULTS

Staining patterns visualized in the human cochlea closely paralleled those seen in other species. Anti-Na,K-ATPase stained strongly the basolateral plasma membrane of strial marginal cells and nerve endings underlying hair cells. This antibody also localized Na,K-ATPase to type II, type IV, and type V fibrocytes in the spiral ligament and in limbal fibrocytes. NKCC was present in the basolateral membrane of strial marginal cells as well as in type II, type V, and limbal fibrocytes. Immunoreactive carbonic anhydrase was present in type I and type III fibrocytes and in epithelial cells lining Reissner's membrane and the spiral prominence.

CONCLUSIONS

The distribution of several major ion transport proteins in the human cochlea is similar but not identical to that described in various rodent models. These results support the presence of a complex system for recycling and regulating K+ homeostasis in the human cochlea, similar to that described in other mammalian species.

Expression pattern of the mouse ortholog of the Pendred's syndrome gene (Pds) suggests a key role for pendrin in the inner ear

Pendred's syndrome is an autosomal-recessive disorder characterized by deafness and goiter. After our recent identification of the human gene mutated in Pendred's syndrome (PDS), we sought to investigate in greater detail the expression of the gene and the function of its encoded protein (pendrin). Toward that end, we isolated the corresponding mouse ortholog (Pds) and performed RNA in situ hybridization on mouse inner ears (from 8 days postcoitum to postnatal day 5) to establish the expression pattern of Pds in the developing auditory and vestibular systems. Pds expression was detected throughout the endolymphatic duct and sac, in distinct areas of the utricle and saccule, and in the external sulcus region within the cochlea. This highly discrete expression pattern is unlike that of any other known gene and involves several regions thought to be important for endolymphatic fluid resorption in the inner ear, consistent with the putative functioning of pendrin as an anion transporter. These studies provide key first steps toward defining the precise role of pendrin in inner ear development and elucidating the pathogenic mechanism for the deafness seen in Pendred's syndrome.

Transforming growth factor-alpha with insulin induces proliferation in rat utricular extrasensory epithelia

Hair cell loss in the human inner ear leads to sensorineural hearing loss and vestibular dysfunction. Recent studies suggest that exogenous addition of growth factors, for example, transforming growth factor-alpha with insulin, may stimulate the production of new supporting cells and hair cells in the mature mammalian vestibular sensory epithelium. Before any growth factor can be seriously considered for the treatment of clinical problems related to hair cell loss, its effects on the extrasensory epithelia must also be fully explored. The aim of this study was to determine whether transforming growth factor-alpha and insulin stimulate cell proliferation in rodent vestibular extrasensory epithelia. The cell proliferation marker, tritiated thymidine, was infused along with transforming growth factor-alpha, insulin, or transforming growth factor-alpha plus insulin into the inner ears of adult rats via osmotic pumps. Effects of the test agents were assessed on normal and drug-damaged utricles. Drug damage was produced by delivering gentamicin directly into the inner ear before the infusion of test agent. Animals were killed 4 or 10 days after pump placement. Utricles were sectioned, processed for autoradiography, and examined for labeled cells within the extrasensory epithelia. In normal animals, transforming growth factor-alpha plus insulin stimulated DNA synthesis in all regions of the extrasensory epithelia, suggesting that these agents are mitogenic for these tissues.

Localization of pH regulating proteins H+ATPase and Cl-/HCO3- exchanger in the guinea pig inner ear

Mechanisms that regulate endolymphatic pH are unknown. It has long been recognized that, because of the large positive endolymphatic potential in the cochlea, a passive movement of protons would be directed out of endolymph leading to endolymphatic alkalization. However, endolymphatic pH is close to that of blood, suggesting that H+ is being secreted into endolymph. Since the kidney and the inner ear are both actively engaged in fluid and electrolyte regulation, we attempted to determine whether proteins responsible for acid secretion in the kidney also exist in the guinea pig inner ear. To that end, a monoclonal antibody against a 31 kDa subunit of a vacuolar vH+ATPase and a polyclonal, affinity purified antibody against the AE2 Cl-/HCO3- exchanger (which can also recognize AE1 under some conditions) were used. In the cochlea, the strongest immunoreactivity for the vH+ATPase was found in apical plasma membranes and apical cytoplasm of strial marginal cells. These cells were negative for the Cl-/HCO3- exchanger. Certain cells of the inner ear demonstrated both apical staining for vH+ATPase and basolateral staining for the Cl-/HCO3- exchanger; these included interdental cells and epithelial cells of the endolymphatic sac. Cochlear cell types with diffuse cytoplasmic staining for vH+ATPase and a basolaterally localized Cl-/HCO3- exchanger included inner hair cells, root cells and a subset of supporting cells in the organ of Corti. Hair cells of the utricle, saccule and cristae ampullaris also expressed both vH+ATPase and the Cl-/HCO3- exchanger, but immunostaining for the vH+ATPase was less intense and less polarized than in the cochlea. These immunocytochemical results support a role for the vH+ATPase and Cl-/HCO3- exchanger in the regulation of endolymphatic pH and suggest that certain cells (including strial marginal cells and epithelial cells of the endolymphatic sac) may be specialized for this regulation.

Evidence for an Na(+)-K(+)-Cl- cotransporter in mammalian type I vestibular hair cells

In amniotes, there are two types of hair cells, designated I and II, that differ in their morphology, innervation pattern, and ionic membrane properties. Type I cells are unique among hair cells in that their basolateral surfaces are almost completely enclosed by an afferent calyceal nerve terminal. Recently, several lines of evidence have ascribed a motile function to type I hair cells. To investigate this, elevated external K+, which had been used previously to induce hair cell shortening, was used to induce shape changes in dissociated mammalian type I vestibular hair cells. Morphologically identified type I cells shortened and widened when the external K+ concentration was raised isotonically from 2 to 125 mM. The shortening did not require external Ca2+ but was abolished when external Cl- was replaced with gluconate or sulfate and when external Na+ was replaced with N-methyl-D-glucamine. Bumetanide (10-100 microM), a specific blocker of the Na(+)-K(+)-Cl- cotransporter, significantly reduced K(+)-induced shortening. Hyposmotic solution resulted in type I cell shape changes similar to those seen with high K+, i.e., shortening and widening. Type I cells became more spherical in hyposmotic solution, presumably as a result of a volume increase due to water influx. In hypertonic solution, cells became narrower and increased in length. These results suggest that shape changes in type I hair cells induced by high K+ are due, at least in part, to ion and solute entry via an Na(+)-K(+)-Cl- cotransporter, which results in cell swelling. A scheme is proposed whereby the type I hair cell depolarizes and K+ leaves the cell via voltage-dependent K+ channels and accumulates in the synaptic space between the type I hair cell and calyx. Excess K+ could then be removed from the intercellular space by uptake via the cotransporter.

In vivo vestibular blood flow in the Mongolian gerbil: angiotensin III-provoked changes in systemic and local factors

The current literature contains little information on vestibular end organ blood flow. The absence of an accepted model, difficulties applying dynamic in vivo measurement techniques and the inaccessibility of the inner ear organs contribute to the shortage of experimental findings. The purpose of the current study is to introduce the gerbil as a viable model for the in vivo study of vestibular blood flow dynamics. The potent vasoactive peptide, angiotensin III (AIII), was used to provoke blood pressure and blood flow changes. The results of this study demonstrate that viable blood flow measures may be obtained from the vestibule of the gerbil. Dose-dependent changes in blood pressure and vestibular blood flow were observed in response to high concentrations of AIII. Pretreatment with the receptor antagonist, sarthran, attenuated both blood pressure and blood flow increases in response to subsequent AIII infusions. The gerbil model offers the advantages of easily accessible and identifiable peripheral vestibular organs, as well as responsive local blood flow. Investigations using this model may provide information on the regulation of blood flow during presentation with a variety of stimulus modalities. Information from such studies may lead to development of strategies for treatment of vestibulopathies suspected to be of vascular origins.

Transmission between the type I hair cell and its calyx ending

The long, uninterrupted apposition between the type I hair cell and the calyx ending has implications for the intercellular communication between these structures. Conventional synaptic transmission will be compromised unless the impedance of the ending is made relatively high. The apposition also creates the possibility of ephaptic transmission between the hair cell and the ending. Ephaptic transmission from the hair cell to the outer face of the calyx ending is too weak to make more than a minor contribution to sensory coding. Basolateral currents associated with hair-cell transduction can result in a substantial accumulation of K+ ions in the intercellular space. The accumulation can alter conventional transmission by depolarizing the hair cell and can alter afferent firing by depolarizing the ending. Reasons were presented suggesting that K+ accumulation may play an essential role in transduction involving type I hair cells, including the linearization of input-output relations and an increase in the maximal rate of discharge.

Differentiation of glycoconjugates localized to sensory terminals and selected sites in brain

Distribution of complex carbohydrates in the peripheral and central nervous systems was investigated cytochemically with a lectin that binds specifically to terminal alpha GalNAc and with monoclonal antibodies against carbohydrate epitopes, including glucuronic acid 3-SO4 and chondroitins 6-SO4 and 4-SO4. Comparative staining with these methods differentiated and partially characterized several glycoconjugates in various sites and allowed a comparison of chemical heterogeneity to neural specialization. Distal terminals of sensory neurons concerned with hearing, balance, taste, touch, and sight expressed glucuronyl 3-SO4, which apparently was present in an undefined glycoprotein. Some neurons in sensory nuclei of the brainstem exhibited a similar constituent on their surfaces. Retinal rod outer segments and the cerebellar granular layer possessed masked glucuronyl 3-SO4 that became immunopositive after digestion with chondroitinase ABC and that occurred in chondroitin 6-SO4 and chondroitin 4-SO4, respectively. The surface of neurons in the eighth nerve root and in neighboring nodes of Ranvier stained for unmasked glucuronic acid 3-SO4 and chondroitin 6-SO4. Some neurons of the cerebral cortex expressed unmasked glucuronyl 3-SO4, chondroitin 6-SO4, and terminal alpha GalNAc on their surfaces. Certain cortical neurons and nerve tracts with chondroitin 6-SO4 and terminal alpha GalNAc lacked glucuronyl 3-SO4, and other neurons possessing chondroitin 6-SO4 failed to express either glucuronyl 3-SO4 or terminal alpha GalNAc. Lability of lectin affinity to hyaluronidase suggested the presence of terminal alpha GalNAc in the chondroitin 6-SO4 on cortical neurons. The findings document further the heterogeneity of neural glycoconjugates, expand knowledge about the diversity of neurons with respect to their content of partially characterized glycoconjugates, and link glucuronyl 3-SO4 with or without chondroitin 6-SO4 spatially to sites of active Na+ transport in sensory nerves.

Expression of Na+/myo-inositol cotransporter mRNA in the inner ear of the rat

We have demonstrated the cellular localization of Na+/myo-inositol cotransporter (SMIT) mRNA in the rat inner ear by in situ hybridization. In the cochlea, the most intense SMIT mRNA signals were observed in fibrocytes of the spiral ligament, moderate signals were found in the spiral limbus, inner hair cells and spiral ganglion cells, while the hybridization signals were almost undetectable in the marginal cells of the stria vascularis and outer hair cells. In the vestibular system, moderate hybridization signals were found in the sensory epithelium, fibrocytes and vestibular ganglion cells. These findings suggest that SMIT plays an important role in maintenance of intracellular ionic balance and cell volume in the inner ear, especially in the fibrocytes associated with generation of the ion gradients between the endolymph and perilymph.

Immunolocalization of aquaporin CHIP in the guinea pig inner ear

Aquaporin CHIP (AQP-CHIP) is a water channel protein previously identified in red blood cells and water transporting epithelia. The inner ear is an organ of hearing and balance whose normal function depends critically on maintenance of fluid homeostasis. In this study, AQP-CHIP, or a close homologue, was found in specific cells of the inner ear, as assessed by immunocytochemistry with the use of affinity-purified polyclonal antibodies against AQP-CHIP.AQP-CHIP was predominantly found in fibrocytes in close association with bone, including most of the cells lining the bony labyrinth and in fibrocytes lining the endolymphatic duct and sac. AQP-CHIP-positive cells not directly apposing bone include cells under the basilar membrane, some type III fibrocytes of the spiral ligament, fibrocytes of the spiral limbus, and the trabecular perilymphatic tissue extending from the membranous to the bony labyrinth. AQP-CHIP was also found in the periosteum of the middle ear and cranial bones, as well as in chondrocytes of the oval window and stapes. The distribution of AQP-CHIP in the inner ear suggests that AQP-CHIP may have special significance for maintenance of bone and the basilar membrane, and for function of the spiral ligament.

Osteopontin expression detected in adult cochleae and inner ear fluids

Localization of protein epitopes and mRNA expression showed that there was a wide-spread distribution of osteopontin (OPN) within the membranous labyrinth of the adult mammalian cochleae. Immunoreaction product and mRNA were found within the stria vascularis, VIIIth cranial nerve, spiral ligament and limbus. Only specific cell types within these regions contained abundant OPN mRNA or protein, the main cell type being fibrocytes that populate the spiral limbus and spiral ligament. Epithelial cells that line the luminal surface of the stria vascularis (marginal cells) and neurons that compose the vestibular and auditory ganglia also showed high opn expression. The pattern of anti-OPN staining within membranous labyrinth was comparable to that observed in tissues such as gall bladder, breast and kidney. In those tissues, luminal epithelial cells, corresponding to the marginal cells of the stria vascularis, may be responsible for manufacturing and secreting OPN into the luminal fluids. consistent with those observations, we detected OPN epitopes in cochlear fluids withdrawn from the scalae media and tympani of the cochlea. We found that the protein species in cochlear fluid differed from those present in cerebrospinal fluid (CSF) suggesting that OPN exists in tissue-specific isoforms that may correspond to particular cellular functions.

Gap junction systems in the rat vestibular labyrinth: immunohistochemical and ultrastructural analysis

The distribution of gap junctions within the vestibular labyrinth was investigated using immunohistochemistry and transmission electron microscopy. Connexin26-like immunoreactivity was observed among supporting cells in each vestibular sensory epithelium. Reaction product was also present in the transitional epithelium of each vestibular endorgan and in the planum semilunatum of crista ampullaris. No connexin26-like immunoreactivity was observed among thin wall epithelial cells or among vestibular dark cells. In addition, fibrocytes within vestibular connective tissue were positively immunostained. Reaction product was also detected in the melanocyte area just beneath dark cells. Ultrastructural observations indicated that a gap junction network of vestibular supporting cells extends to the transitional epithelium and planum semilunatum and forms an isolated epithelial cell gap junction system in each vestibular endorgan. In contrast, no gap junctions were found among wall epithelial cells or among dark cells. Fibrocytes and melanocytes were coupled by gap junctions and belong to the connective tissue cell gap junction system, which is continuous throughout the vestibular system and the cochlea. The possible functional significance of these gap junction systems is discussed.

Expression of alpha and beta subunit isoforms of Na,K-ATPase in the mouse inner ear and changes with mutations at the Wv or Sld loci

Mice homozygous for mutations at the viable dominant spotting (Wv) and Steel-dickie (Sld) loci exhibit a similar phenotype which includes deafness. The auditory dysfunction derives from failure of the stria vascularis to develop normally and to generate a high positive endocochlear potential (EP). Because strial function is driven by Na,K-ATPase its expression was investigated in inner ears of Wv/Wv and Sld/Sld mice and their wild-type littermates by immunostaining with antisera against four of the enzyme's subunit isoforms. Wild-type mice from two different genetic backgrounds showed an identical distribution of subunit isoforms among inner ear transport cells. Several epithelial cell types coexpressed the alpha 1 and beta 1 subunits. Vestibular dark cells showed no reactivity for beta 1 but expressed abundant beta 2, whereas, strial marginal cells stained strongly for both beta isoforms. The only qualitative difference between mutant and wild-type mice was the absence of beta 1 subunit in marginal cells of the mutant's stria. However, it is unlikely that this difference accounts for failure of mutants to generate a high EP because the beta 1 subunit is not present in the stria vascularis of either rats or gerbils with normal EP values. Strong immunostaining for Na,K-ATPase in lateral wall fibrocytes of normal mice along with diminished immunoreactivity in the mutants supports the concept that these strategically located transport fibrocytes actively resorb K+ leaked across Reissner's membrane into scala vestibuli or effluxed from hair cells and nerves into scala tympani. It is further speculated that the resorbed K+ normally is siphoned down its concentration gradient into the intrastrial space through gap junctions between fibrocytes and strial basal and intermediate cells where it is recycled back to endolymph via marginal cells. Thus, failure of mutants to generate a positive EP could be explained by the absence of intermediate cells which may form the final link in the conduit for moving K+ from perilymph to the intrastrial compartment.

Na,K-ATPase alpha and beta subunit isoform distribution in the rat cochlear and vestibular tissues

The distribution of five Na,K-ATPase subunit isoforms (alpha 1, alpha 2, alpha 3, beta 1 and beta 2) in rat cochlear and vestibular tissues was determined by immunocytochemical techniques using subunit isoform specific polyclonal antibodies. The expression of Na,K-ATPase alpha and beta subunit isoforms varied among different cell regions of the inner ear. The alpha 1 subunit isoform was more extensively distributed in all inner ear tissues than the alpha 2 or alpha 3 subunit isoforms. The beta 1 subunit isoform was distributed primarily in spiral ligament and inner hair cells of the cochlea, and in crista ampullaris and macula of the saccule. The beta 2 subunit isoform was most abundant in the stria vascularis, dark cells of the ampullae and utricle. The alpha 1 beta 1 subunit combination of Na,K-ATPase was most commonly found in the spiral ligament, while the alpha 1 beta 2 combination was most abundant in the stria vascularis. Similarly, alpha 1 beta 2 was confined more to the dark cells of the ampullae and utricle. The alpha 3 beta 1 subunit combination of Na,K-ATPase was identified in the inner hair cells of the cochlea and the sensory regions of the vestibular end organs. These observations may reflect functional diversity of Na,K-ATPase in the individual inner ear regions and may provide insight into the differences between fluid and ion transport in the inner ear and that of other transporting tissues. Overall, the distribution pattern further indicates that the different isoform combinations have specific roles.

Immunolocalization of Na+, K(+)-ATPase, Ca(++)-ATPase, calcium-binding proteins, and carbonic anhydrase in the guinea pig inner ear

The distribution of Na+, K(+)-ATPase, Ca(++)-ATPase, carbonic anhydrase, and calcium-binding proteins were investigated immunohistochemically in paraffin sections of guinea pig inner ears. Marginal cells of the stria vascularis, type II fibrocytes of the spiral ligament, and cells in supralimbal and suprastrial regions, were positive for Na+, K(+)-ATPase. Type I fibrocytes of the spiral ligament were positive for Ca(++)-ATPase, carbonic anhydrase, calmodulin and osteopontin. In the vestibular system, dark cells were positive for Na+, K(+)-ATPase. However, these cells and subepithelial fibrocytes were negative for Ca(++)-ATPase, carbonic anhydrase, and the calcium-binding proteins. In the endolymphatic sac, epithelial cells in intermediate and distal portions were positive for Na+, K(+)-ATPase, but the reaction was less than that in the stria. The same endolymphatic sac cells that were positive for Na+, K(+)-ATPase were also positive for Ca(++)-ATPase and calcium-binding proteins, but negative for carbonic anhydrase. The presence of Ca(++)-ATPase and calcium-binding proteins in the type I fibrocytes of the spiral ligament suggests that these cells are involved in mediating Ca++ regulation. Lower levels of Na+, K(+)-ATPase and the co-existence of Ca(++)-ATPase and calcium-binding proteins in the epithelial cells of the endolymphatic sac indicate that these cells have a distinctive role in ion transport that is different from that of the cells of the stria vascularis and vestibular dark cells.

Ba2+ and amiloride uncover or induce a pH-sensitive and a Na+ or non-selective cation conductance in transitional cells of the inner ear

The membrane potential Vm the cytosolic pH (pHi), the transference numbers (t) for K+, Cl- and Na+/non-selective cation (NSC) and the pH-sensitivity of Vm were investigated in transitional cells from the vestibular labyrinth of the gerbil. Vm, pHi, tK+, tCl-, tNa+/NSC, and the pHi sensitivity of Vm were under control conditions were -92 +/- 1 mV (n = 89 cells), pHi 7.13 +/- 0.07 (n = 11 epithelia), 0.87 +/- 0.02 (n = 22), 0.02 +/- 0.01 (n = 19), 0.01 +/- 0.01 (n = 24) and -5 mV/pH unit (n = 13 cells/n = 11 epithelia), respectively. In the presence of 100 mumol/l Ba2+ the corresponding values were: -70 +/- 1 mV (n = 32), pHi 7.16 +/- 0.08 (n = 6), 0.31 +/- 0.05 (n = 4), 0.06 +/- 0.01 (n = 6), 0.20 +/- 0.03 (n = 10) and -16 mV/pH-unit (n = 15/n = 6). In the presence of 500 mumol/l amiloride the corresponding values were: -72 +/- 2 mV (n = 34), pHi 7.00 +/- 0.07 (n = 5), 0.50 +/- 0.04 (n = 6), 0.04 +/- 0.01 (n = 11), 0.28 +/- 0.04 (n = 9) and -26 mV/pH-unit (n = 20/n = 5). In the presence of 20 mmol/l propionate plus amiloride the corresponding values were: -61 +/- 2 mV (n = 27), pHi 6.72 +/- 0.06 (n = 5), 0.30 +/- 0.02 (n = 6), 0.06 +/- 0.01 (n = 5) and 0.40 +/- 0.02 (n = 8), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical localization of intracellular Ca-ATPase in outer hair cells, neurons and fibrocytes in the adult and developing inner ear

Intracellular isoforms of the enzyme Ca-ATPase were identified in the inner ear by immunostaining paraffin sections with a polyclonal antiserum against rabbit cardiac muscle Ca-ATPase. In the adult cochlea, intense staining was present at the lateral border of outer hair cells in regions corresponding with the distribution of the subsurface cisternal system. Other cell types containing high levels of Ca-ATPase were skeletal muscle fibers in the tensor tympani, vascular smooth muscle, spiral ganglion neurons and subpopulations of fibrocytes in the limbus, spiral ligament and underlying vestibular neurosensory epithelium. In neonatal gerbils, staining of tensor tympani muscle fibers was observed at 4 days after birth and approached adult levels by 8 days after birth. Ca-ATPase was first detected in other cell types between postnatal days 12 and 14 but immunostaining still remained well below the intensity seen in adults at 20 days after birth. The demonstration of abundant calcium pumps in the subsurface cisternae confirms the role of this organelle as an intracellular reservoir for Ca2+ in outer hair cells. The presence of high levels of Ca-ATPase in spiral ganglion neurons and in fibrocytes specialized for ion transport points to a role for the enzyme in regulating the activity of other cell types of importance to normal hearing.

Quantitative immunoelectron microscopic localization of Na, K-ATPase alpha-subunit in the epithelial cells of rat vestibular apparatus

Na, K-ATPase was quantitatively localized in the epithelial cells of rat vestibular apparatus such as macula utriculi, macula sacculi and crista ampullaris. Immunogold localization method was carried out at the saturation level of antibody using an affinity purified antibody against the alpha-subunit of rat kidney Na, K-ATPase. Numerous gold particles were found on the basolateral membrane of the dark cells, a small number of gold particles were found on the basolateral membrane of the transitional epithelium cells and hair cells, but the luminal surface membranes of the hair cells, transitional epithelium cells, planum semilunatum cells and dark cells were rarely labeled by gold particles. Significance of the abundant localization of Na, K-ATPase on the basolateral surface of the dark cells in the production of endolymph was discussed.

Differentiation of inner ear fibrocytes according to their ion transport related activity

Fibrocytes in the lateral wall and limbus of the gerbil cochlea evidenced a capacity for ion transport activity by immunostaining for transport mediating enzymes including Na,K-ATPase, carbonic anhydrase (CA) and creatine kinase (CK). Fibrocytes of the spiral ligament unlike those in the suprastrial region and limbus decreased in abundance from base to apex. Spiral ligament fibrocytes at a given position along the cochlea varied in content of transport related enzymes, and on the basis of immunostaining, location and orientation, were classified into four types. Type I fibrocytes under the stria vascularis stained for CA isozymes II and III and CK isozyme BB. Type II fibrocytes under the outer sulcus and spiral prominence epithelium were found to contain only Na,K-ATPase. Type III fibrocytes lying adjacent to bone in the inferior region of the spiral ligament contained CA II and III and CK isozymes BB and MM. Type IV fibrocytes located more superficially in the inferior part of the spiral ligament stained variably for all the enzymes. Superficial fibrocytes in the suprastrial area disclosed Na,K-ATPase whereas the underlying fibrocytes stained for CA and CK. Limbal fibrocytes reacted with antisera to all the enzymes except CA III. Most fibrocytes in stromal plates beneath the vestibular system's neurosensory epithelium contained Na,K-ATPase and CA II but not CA III. These findings point to cooperativity in fluid and ion transport between epithelial cells and neighboring fibrocytes and demonstrate functional diversity of fibrocytes of the inner ear providing a basis for classifying those in the spiral ligament.

Immunocytochemical demonstration of Na+,K(+)-ATPase in internodal axolemma of myelinated fibers of rat sciatic and optic nerves

We used postembedding electron microscopic immunocytochemistry with colloidal gold to determine the ultrastructural distribution of Na+,K(+)-ATPase in the sciatic and optic nerves of the rat. Using a polyclonal antiserum raised against the denatured catalytic subunit of brain Na+,K(+)-ATPase, we found immunoreactivity along the internodal axolemma of myelinated fibers in both nerves. This antiserum did not produce labeling of nodal axolemma. These results suggest that an important site of energy-dependent sodium-potassium exchange is along the internodal axolemma of myelinated fibers in the mammalian CNS and PNS and that there may be differences between the internodal and nodal forms of the enzyme.

Differential distribution of (Na, K)-ATPase alpha isoforms in the central nervous system

1. mRNA transcripts for three isoforms of the alpha subunit of (Na,K)-ATPase have been previously identified in the rat nervous system and designated alpha 1, alpha 2 and alpha 3. 2. In order to study the localization and expression of the different alpha isoform mRNAs on a regional and cellular level in the brain, we prepared probes from the unique 3' untranslated region of rat alpha 1 cDNA and from a segment containing a portion of the translated region of rat alpha 3 cDNA. These probes were used in dot blot and in situ hybridization assays of rat brain. 3. alpha 1 mRNA was found predominantly in cerebral cortex, dentate gyrus of hippocampus, and specific isolated brain-stem nuclei such as locus ceruleus and motor nuclei V and VII. In contrast alpha 3 mRNA was found predominantly in pyramidal neurons in the deep layers of cerebral cortex, in both pyramidal and dentate gyrus neurons of the hippocampus, and in neurons of most subcortical structures of the thalamus, basal ganglia, and brain-stem nuclei. 4. In the cerebellum, Purkinje cells showed predominantly alpha 3, as did stellate and basket cells. The granule cells contained predominantly alpha 1. 5. These experiments show that mRNAs for both alpha 1 and alpha 3 isoforms of (Na,K)-ATPase are found in neurons of the CNS. The isoforms have unique cellular and regional distributions, which in some cases overlap.